Nanoparticles, i.e. particles with physical size (diameter) of about 100 nanometers (nm) diameter or less, typically less than 50 nm diameter, possess important technological properties ranging from superior mechanical behavior to novel electronic and magnetic properties by virtue of their nanocrystalline or other nano-scale microstructural features. Unfortunately, nanoparticles, by virtue of their size and high surface area, are very reactive and interact with their surroundings quickly. For example, metal nanoparticles tend to oxidize rapidly when exposed to air. The desirability of the encapsulation of nanoparticles of metals inside graphite shells has been recognized. For instance, magnetic materials (such as metallic nickel, iron, cobalt or cobalt-chromium alloy) encapsulated inside graphite shells can find applications as recording media, ferro fluids or magnetic tagging elements. These nanocapsules may be injected into biological systems for use as a drug or a tracing delivery and monitoring system. These particulate materials may also find applications in electronic and optoelectronic industries by virtue of their small particle size, which would give rise to novel quantum phenomena. The encapsulated nanoparticles may be consolidated or dispersed in a matrix to form interpenetrated composites which will have applications in areas which require better mechanical properties or unique electronic and magnetic properties.
Generally the prior art synthesis methods utilize an arc between two graphite electrodes in which one electrode (the anode) is a mixture of graphite and the material to be encapsulated, and the other electrode (the cathode) is graphite. Such a process generally results in isolated instances of encapsulation with a low yield.
Furthermore, the process also produces a lot of empty graphite shells, graphite flakes, amorphous debris and graphite nanotubes which are difficult to separate from those which encapsulate the material of interest.
Other methods suffer from surface contamination by impurities, which can greatly change the desired properties of a bulk sample. The inert gas vapor condensation method is one of the cleanest ways to produce nanophase materials. Because the material is physically evaporated in the absence of any precursors, there are no contaminants left on the surface of the nanoparticles, which eliminates a common problem with chemical methods. However, typically either the particle size is too large, the production rate is too low, or both using these techniques. Heating methods include electrical resistance heaters, arc discharge, laser heating, and electron beam heating. In traditional resistance heated evaporators, the vapor is formed in a partial vacuum above an open crucible or boat and allowed to rise from the molten pool by convection. Low evaporation rates and low pressures are required to yield small particles, resulting in low rates of production of nanoparticles. In the arc heating method, a flow of gas at atmospheric pressure is passed through the arc chamber to carry the condensed particles away from the arc, but the average particle size tends to be substantially large with respect to nanoparticle range; e.g. larger than 20 nm.
Traditional evaporators also suffer from the disadvantage that two metals with disparate vapor pressures cannot be vaporized simultaneously at the arbitrary relative rates required to form alloy, intermetallic, or other compound particles of specific composition. Even when two metals are evaporated simultaneously by two heating devices in the same chamber, the two vapors condense separately such that composition control is difficult, and the resulting particles are a mixture of two particle types of different composition. Electron beam heating has been utilized in an attempt to evaporate two metals simultaneously, but the results have been sporadic with control of composition being difficult and imprecise.
The production of oxide nanosize powder particles from metal vapors traditionally has been effected by first collecting the metal nanoparticles and then allowing them to oxidize on the collector. Among other problems, the oxidized particles are agglomerated as a result of the metal nanoparticles being agglomerated prior to oxidation.
Copending application Ser. No. 330 326 of common assignee herewith describes a method of encapsulating nanoparticles in a graphite shell that overcomes the shortcomings and problems experienced by the prior art discussed hereabove. The method employs a tungsten arc method wherein an arc chamber is filled with an inert or reducing gas and a tungsten rod is used as a non-consumable cathode. The anode comprises the material, such as a metal, alloy, etc., which is to be formed into nanoparticles. The anode material is contained in a graphite crucible and is vaporized by the electric arc established between the anode and the non-consumable cathode. The high temperature vapor of the anode material is rapidly quenched by directing a jet of helium or other inert gas transversely through the electric arc and vapor plume produced thereby. The rapid quenching of the vapor produces nanoparticles coated with graphite that are collected for further processing.
An object of the present invention is to provide for the production of nanoparticles by quench condensation of a high temperature vapor generated and discharged by an evaporator having features effective to isolate evaporation conditions from downstream conditions.
A further object of the present invention is to provide for the production of nanoparticles of controlled, multi-element composition by quench condensation of a high temperature vapor generated by an evaporator capable of concurrent evaporation of materials of dissimilar or similar vapor pressures.
Still a further object of the subject invention is to provide for the production of nanoparticles comprising ceramic and other compounds by quench condensation of a high temperature vapor generated by an evaporator and discharged as a jet to a reactive atmosphere reactive with the evaporated element(s) present in the high temperature vapor.